The transmission of radio signals carrying data in modern wireless communications can be realized based on the number of different communication systems, often specified by a standard. Mobile radio receiver devices include analog radio frequency (RF)/intermediate frequency (IF) stages which are arranged to receive and transmit wireless signals via one or more antennas. The output of the RF/IF stages is typically converted to baseband, wherein an analog to digital converter (ADC) converts incoming analog signals to digital samples, which are then processed for signal detection and decoding of the data in the form of reliability values. The ADC may alternatively operate directly at IF, in which case the conversion to baseband is performed in the digital domain.
An important factor in processing the digital samples is given by the knowledge of the signal to interference ratio (SIR) on the transmission channel or channels which have been used for the signal. One way of estimating the SIR is to use the CPICH (common pilot channel) to derive a channel estimate, and then use the estimated channel to evaluate the SIR. In that case it is necessary to correct for possible power differences in the transmission of the common pilot channel and the data channel.
In a 3GPP wideband code division multiple access (WCDMA) receiver, the downlink dedicated physical channel (DPCH) carries pilot symbols that can be used to evaluate the signal-to-interference ratio (SIR) of the DPCH data channel (DPDCH) at the output of the signal detector. Signal detection can be based for instance on rake processing (per finger de-scrambling and de-spreading followed by fingers combining) or chip-level equalization (equalizer filtering followed by de-scrambling and de-spreading). Compared to the above-described alternative of using CPICH channel estimate to evaluate the SIR and correcting for channel power differences, the use of the dedicated pilots at the detector output has the advantage of simplicity and allows to take in account any imperfections introduced by a specific implementation of signal detection of the DPCH data.
In the case of a single transmit antenna, both the pilots and the data are coded in the same way at the transmitter. This allows the use of the DPCH pilots to estimate the SIR of DPDCH symbols without any special processing on the pilot symbols. However, in the case of close loop transmit diversity (CLTD) the pilot and the data symbols are coded differently, as described in 3GPP TS 25.211, “Technical Specification Group Radio Access Network, Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD)”, December 2005, Section 5.3.
FIG. 1 is a schematic block diagram of the transmitter processing, illustrating the use of multiple transmit antennas, and the transmitter CLTD processing for the DPCH channel.
Two transmit antennas A1, A2 are shown in FIG. 1, though it is possible for there to be more than two transmit antennas. The antennas transmit wireless signals corresponding to first and second pilot sequences labeled ‘DPCH Pilots 1’ and ‘DPCH Pilots 2’, and the data sequence labeled ‘DPCH Data’. Symbols from each of the sequences are applied respectively to the multipliers M1, M3 and M2, which multiply the symbols by the scrambling/spreading code in a manner which is known per se. The resulting chip sequences obtained from the first pilot sequence ‘DPCH Pilots 1’ and the data sequence ‘DPCH Data’ are supplied to a first slot multiplexer SM1, while the chip sequences obtained from the second pilot sequence ‘DPCH Pilots 2’ and the data sequence ‘DPCH Data’ are supplied to a second slot multiplexer SM2. These allow the data and pilot information to be carried according to the specified slot format in a DPCH slot, in a manner known per se. The outputs of the slot multiplexers are weighted using weighting factors w1, w2 respectively, and then supplied to the transmitter antennas A1, A2 for transmission.
Details on the specific pilot symbols structure used on the DPCH channel are shown in FIG. 2, for the three different pilot lengths Np=2, 4 and 8. These pilot sequences are orthogonal across the transmit antennas. The pilot symbol s is constant, s=(1+j)/√{square root over (2)}. The other pilot symbols in FIG. 2 are QPSK symbols that change from one slot to another as specified in Table 12 and Table 15 of 3GPP TS 25.211, “Technical Specification Group Radio Access Network; Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD)”, December 2005, Section 5.3.
The pilot sequence shown in FIG. 2 is composed of Np/2 orthogonal symbol pairs:                For Np=2, the orthogonal pair is derived from the sequence itself.        For Np=4, the 2 orthogonal pairs correspond to time indices (0,2) and (1,3).        For Np=8, the 4 orthogonal pairs correspond to time indices (0,2), (1,3), (4,6) and (5,7).        
FIG. 3 is a schematic block diagram of the receiver signal processing. Antenna A3 denotes the antenna for receiving the wireless signal which is processed by the receiver front-end, converted to baseband, and supplied to the signal detection block 2. At the detector output, the dedicated physical channel is supplied to a slot demultiplexer 4 which demultiplexes the pilot symbols separately from the data symbols. The signal samples corresponding to the pilot symbols are supplied to an SIR estimation block 6, while the samples corresponding to the data symbols are supplied to an LLR calculation/demapping block 8 which generates bit reliability values. In order to do this, the LLR calculation/demapping block 8 uses the estimated signal power and noise power values from the SIR estimation block 6. The signal detection could be based on rake processing or equalizer processing, and the antenna weights w1, w2 are handled herein in a manner known per se.
At the output of the DPCH slot demultiplexer 4 of FIG. 3, the DPCH pilot signal for the pilot index k (k=0, . . . , Np−1, with Np number of pilot symbols) can be written asyp(k)=h1s1(k)+h2s2(k)+n(k),  Equation 1
where s1(k), E{|s1(k)|2}=1 (respectively s2(k), E{|s2(k)|2}=1) denotes the pilot symbol on antenna 1 (respectively antenna 2), h1 (respectively h2) represents the channel gain from antenna 1 (respectively antenna 2) and n(k) is an additive noise process that models the noise plus interference at the output of the signal detection stage. In equation 1, the CLTD antenna weights w1, w2 are included in the channel gains h1, h2.
The DPDCH signal at the output of the DPCH slot demultiplexer of FIG. 3 is written asyd(k)=(h1+h2)·d(k)+n(k),  Equation 2
where d(k) is the transmitted data stream, E{|d(k)|2}=1. In contrast to the pilot symbols, the data symbols experience the composite channel of antenna 1 and 2, h=h1+h2.
The SIR on the DPDCH signal is therefore
                              SIR          =                                                    P                S                                            P                N                                      =                                                            (                                      1                    /                    γ                                    )                                ⁢                                                                                                                        h                        1                                            +                                              h                        2                                                                                                  2                                                            P                N                                                    ,                            Equation        ⁢                                  ⁢        3            
where γ is the power ratio between the pilot and the data transmitted on the DPCH. In the 3GPP WCDMA standard, γ is signaled by the Node-B (by the network) to the UE receiver over a logical control channel.
For the decoding of the data, both the numerator and denominator of the SIR (3) (the signal power PS and the noise power PN) are required, and have to be estimated. Since the DPCH pilot signal has a different structure with respect to the data and is composed of two pilot streams, one per antenna, SIR estimation requires special processing of the pilot signal.
It has been observed that the orthogonal DPCH pilot leads to errors in the SIR measurement when traditional SIR estimation algorithms are used, and therefore it has been suggested to use the data symbols for SIR estimation. A known algorithm described in A. U. Priantoro, M. Okada and H. Yamamoto, “Comparison of SIR-based Closed Loop TPC in W-CDMA Considering Closed Loop Transmit Diversity Mode 1”, IEEE Region 10 Conference, TENCON 2004, vol. 2, November 2004, pp. 525-528, and in A. U. Priantoro, M. F. Mohamad, M. Okada and H. Yamamoto, “Data-aided SIR measurement for closed loop fast TPC suitable for W-CDMA with closed loop transmit diversity,” IEEE International Conference on Personal Wireless Communications, ICPWC 2005, January 2005, pp. 169-173, performs tentative decisions on the data symbol in order to be able to generate estimates of the signal and noise power. This algorithm requires additional complexity due to the need to perform tentative decisions, and suffers from performance degradation under low SIR conditions, where the tentative decisions are less reliable.
It is an aim of the present invention to allow the pilot symbols to be used for SIR estimation, according to a procedure that allows to efficiently compute an estimate of the signal and noise power and of the SIR.